<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0202</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2006-12-26</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0001171014</issuerCik>
        <issuerName>COSI INC</issuerName>
        <issuerTradingSymbol>COSI</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001212718</rptOwnerCik>
            <rptOwnerName>VARDON CAPITAL MANAGEMENT LLC</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>120 WEST 45TH STREET</rptOwnerStreet1>
            <rptOwnerStreet2>17TH FLOOR</rptOwnerStreet2>
            <rptOwnerCity>NEW YORK</rptOwnerCity>
            <rptOwnerState>NY</rptOwnerState>
            <rptOwnerZipCode>10036</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001122016</rptOwnerCik>
            <rptOwnerName>VARDON PARTNERS LP</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>120 WEST 45TH STREET</rptOwnerStreet1>
            <rptOwnerStreet2>17TH FLOOR</rptOwnerStreet2>
            <rptOwnerCity>NEW YORK</rptOwnerCity>
            <rptOwnerState>NY</rptOwnerState>
            <rptOwnerZipCode>10036</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001157866</rptOwnerCik>
            <rptOwnerName>VARDON PARTNERS II LP</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>120 WEST 45TH STREET</rptOwnerStreet1>
            <rptOwnerStreet2>17TH FLOOR</rptOwnerStreet2>
            <rptOwnerCity>NEW YORK</rptOwnerCity>
            <rptOwnerState>NY</rptOwnerState>
            <rptOwnerZipCode>10036</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001349542</rptOwnerCik>
            <rptOwnerName>VARDON INTERNATIONAL BP LTD</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>P O BOX 32021 SMB 2ND FILE</rptOwnerStreet1>
            <rptOwnerStreet2>ANCHORAGE CENTER</rptOwnerStreet2>
            <rptOwnerCity>GRAND CAY CAY ISL BWI</rptOwnerCity>
            <rptOwnerState>E9</rptOwnerState>
            <rptOwnerZipCode>00000</rptOwnerZipCode>
            <rptOwnerStateDescription>CAYMAN ISLANDS</rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001178754</rptOwnerCik>
            <rptOwnerName>VARDON FOCUS FUND LP</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>120 WEST 45TH STREET</rptOwnerStreet1>
            <rptOwnerStreet2>17TH FLOOR</rptOwnerStreet2>
            <rptOwnerCity>NEW YORK</rptOwnerCity>
            <rptOwnerState>NY</rptOwnerState>
            <rptOwnerZipCode>10036</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock, par value $.01 per share</value>
            </securityTitle>
            <transactionDate>
                <value>2006-12-26</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>S</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>2970</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>5.1093</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>38193</value>
                    <footnoteId id="F1"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock, par value $.01 per share</value>
            </securityTitle>
            <transactionDate>
                <value>2006-12-26</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>S</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>57846</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>5.1093</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>23466</value>
                    <footnoteId id="F2"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock, par value $.01 per share</value>
            </securityTitle>
            <transactionDate>
                <value>2006-12-26</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>S</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>1077</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>5.1093</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>84057</value>
                    <footnoteId id="F3"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock, par value $.01 per share</value>
            </securityTitle>
            <transactionDate>
                <value>2006-12-26</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>S</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>4285</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>5.1093</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>1537511</value>
                    <footnoteId id="F4"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock, par value $.01 per share</value>
            </securityTitle>
            <transactionDate>
                <value>2006-12-26</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>S</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>9476</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>5.1093</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>723533</value>
                    <footnoteId id="F5"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock, par value $.01 per share</value>
            </securityTitle>
            <transactionDate>
                <value>2006-12-26</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>J</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>0</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>1882640</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                    <footnoteId id="F6"/>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>See Footnote 6</value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock, par value $.01 per share</value>
            </securityTitle>
            <transactionDate>
                <value>2006-12-26</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>J</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>0</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>4303940</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                    <footnoteId id="F7"/>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>See Footnote 7</value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">These securities are owned by Vardon Partners, L.P., which is a Reporting Person and may be deemed to be a member of a group with Vardon Partners II, L.P., Vardon Focus Fund, L.P., Vardon Focus Fund II, L.P., Vardon Hybrid Fund, L.P. (as of January 1, 2007 known as Vardon Continuum Fund, L.P), Vardon International, Ltd., Vardon International BP, Ltd., Vardon Focus Fund International, Ltd., Vardon Focus International BP, Ltd., Vardon Capital, LLC, Vardon Capital Management, LLC and Richard W. Shea, Jr. for purposes of Section 13(d) of the Exchange Act.</footnote>
        <footnote id="F2">These securities are owned by Vardon Partners II, L.P., which is a Reporting Person and may be deemed to be a member of a group with Vardon Partners, L.P., Vardon Focus Fund, L.P., Vardon Focus Fund II, L.P., Vardon Hybrid Fund, L.P. (as of January 1, 2007 known as Vardon Continuum Fund, L.P), Vardon International, Ltd., Vardon International BP, Ltd., Vardon Focus Fund International, Ltd., Vardon Focus International BP, Ltd., Vardon Capital, LLC, Vardon Capital Management, LLC and Richard W. Shea, Jr. for purposes of Section 13(d) of the Exchange Act.</footnote>
        <footnote id="F3">These securities are owned by Vardon International BP, Ltd., which is a Reporting Person and may be deemed to be a member of a group with Vardon Partners, L.P., Vardon Partners II, L.P., Vardon Focus Fund, L.P., Vardon Focus Fund II, L.P., Vardon Hybrid Fund, L.P. (as of January 1, 2007 known as Vardon Continuum Fund, L.P), Vardon International, Ltd., Vardon Focus Fund International, Ltd., Vardon Focus International BP, Ltd., Vardon Capital, LLC, Vardon Capital Management, LLC and Richard W. Shea, Jr. for purposes of Section 13(d) of the Exchange Act.</footnote>
        <footnote id="F4">These securities are owned by Vardon Focus Fund, L.P., which is a Reporting Person and may be deemed to be a member of a group with Vardon Partners, L.P., Vardon Partners II, L.P., Vardon Focus Fund II, L.P., Vardon Hybrid Fund, L.P. (as of January 1, 2007 known as Vardon Continuum Fund, L.P), Vardon International, Ltd., Vardon International BP, Ltd., Vardon Focus Fund International, Ltd., Vardon Focus International BP, Ltd., Vardon Capital, LLC, Vardon Capital Management, LLC and Richard W. Shea, Jr. for purposes of Section 13(d) of the Exchange Act.</footnote>
        <footnote id="F5">These securities are held in the accounts of certain managed accounts advised by Vardon Capital Management, LLC. Richard W. Shea, Jr. is the managing member of Vardon Capital Management, LLC. Vardon Capital Management, LLC and Richard W. Shea, Jr. each may be deemed the beneficial owner the securities. Each reporting person disclaims beneficial ownership of the reported securities except to the extent of his or its pecuniary interest therein.</footnote>
        <footnote id="F6">These securities are held in the accounts of private investment vehicles to which Vardon Capital, LLC serves as general partner. Richard W. Shea, Jr. is the managing member of Vardon Capital, LLC. Each reporting person disclaims beneficial ownership of the reported securities except to the extent of his or its pecuniary interest therein.</footnote>
        <footnote id="F7">These securities are held in the accounts of certain private investment vehicles advised by Vardon Capital Management, LLC. Richard W. Shea, Jr. is the managing member of Vardon Capital Management, LLC an investment adviser registered with the U.S. Securities and Exchange Commission under the Investment Advisers Act of 1940, as amended. Each reporting person disclaims beneficial ownership of the reported securities except to the extent of his or its pecuniary interest therein.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>Vardon Partners, L.P., Vardon Partners II, L.P., Vardon Focus Fund, L.P.; by their general partner, Vardon Capital, LLC by its Managing Member, Richard W. Shea, Jr.: /s/ Richard W. Shea, Jr.</signatureName>
        <signatureDate>2007-01-19</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>Vardon International BP, Ltd. by its director, Richard W. Shea, Jr.: /s/ Richard W. Shea, Jr.</signatureName>
        <signatureDate>2007-01-19</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>Vardon Capital, LLC by its Managing Member, Richard W. Shea, Jr.: /s/ Richard W. Shea, Jr.</signatureName>
        <signatureDate>2007-01-19</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>Vardon Capital Management, LLC, by its Managing Member, Richard W. Shea, Jr.: /s/ Richard W. Shea, Jr.</signatureName>
        <signatureDate>2007-01-19</signatureDate>
    </ownerSignature>
</ownershipDocument>
